The present invention is directed to a process for forming electrophotographic images. More specifically, the present invention is directed to a process for forming electrophotographic images wherein the developed transferred image becomes permanently affixed to a substrate in the absence of the fusing or fixing thereof. One embodiment of the invention is directed to a process for forming permanent electrophotographic images that comprises generating, in an electrophotographic imaging apparatus, an electrostatic latent image; developing the image with a liquid developer comprising a colorant, a solvent, and a polymeric material having adhesive properties when wetted with the solvent; transferring the image to a substrate having a coating comprising a polymeric material having adhesive properties when wetted with the liquid developer solvent; and permitting the image to dry on the substrate.
The formation and development of images on the surface of photoconductive material by electrostatic means is well known. For example, U.S. Pat. No. 2,297,691 discloses an electrophotographic imaging process that entails placing a uniform electrostatic charge on a photoconductive insulting layer such as a photoconductor or photoreceptor, exposing the photoreceptor to a light and shadow image to dissipate the charge on the areas of the photoreceptor exposed to the light, and developing the resulting electrostatic latent image by depositing on the image a finely divided electroscopic material known as toner. The toner will normally be attracted to those areas of the photoreceptor which retain a charge thereby forming a toner image corresponding to the electrostatic latent image. This developed image may then be transferred to a substrate such as paper, and subsequently be permanently affixed to the substrate. Typical methods of fixing or fusing the developed image include employment of means such as heat, pressure, a combination of heat and pressure, or other suitable fixing means such as an overcoating or solvent treatment. The present invention, however, provides a process for affixing an electrophotographic image to a substrate with no necessity for a fusing step. Rather, upon transfer of the developed image to the substrate and drying of the developer composition, the image is permanently affixed to the substrate.
Reported methods of affixing developed electrophotographic images to a substrate include the application of heat or pressure to the transferred image. For example, U.S. Pat. No. 3,928,656 discloses a pressure fixable toner in which the resinous material is a weakly crosslinked amorphous polymer. The crosslinks in the polymer can be disrupted or broken by the application of pressure after which the polymer becomes sufficiently soft to be fixed to the substrate by pressure.
Coated electrophotographic substrates are also known. For example, U.S. Pat. No. 4,332,851, discloses a coated paper suitable for use in an electrophotographic process for receiving an image formed by a magnetic brush apparatus with a single component developer. The coating composition comprises a crosslinking agent and a styrene/butadiene latex as well as any desired pigment composition and nonionic emulsifier. According to the patent, paper coated with this composition in amounts of 1 to 5 grams per square meter exhibits superior transferrability and pressure fixability.
Another patent, U.S. Pat. No. 4,259,425, discloses an electrographic recording sheet coated with a polymeric binder to improve toner adhesion. The electrically insulating coating comprises a polymeric binder of a polyvinyl acetal present in an amount of from 40 to 90 parts by weight and a polystyrene or a poly (.alpha.-methylstyrene) present in an amount of from 10 to 60 parts by weight as well as an inert, finely divided pigment present in an amount of up to 500 parts by weight per 100 parts by weight of the polymeric binder. A conductive sheet support coated with this composition exhibits improved toner retention when the developed recording material, which has been fixed by the application of heat or pressure, is subjected to an adhesion test.
Although the known materials and methods for permanently affixing electrophotographic images to substrates are suitable for their intended purposes, a need exists for a process for forming permanent electrophotographic images with no fixing or fusing step. Such a process possesses many advantages, including the enablement of inexpensive electrophotographic imaging systems containing no large, costly fusing devices. In addition, images produced according to the process of the present invention possess and retain a high degree of contrast and resolution; the process is thus suitable for applications such as xeroradiographic medical imaging, which requires high contrast and high resolution images.